Assessing Neural State from Action Potentials

ABSTRACT

The neural health or state of a subject is assessed. A recording is obtained of a compound action potential arising in neural tissue of the subject. The recording is processed to determine whether a profile of the recorded compound action potential is anomalous, such as by exhibiting doublets, peak broadening or deformation, or other anomaly. An indication is output regarding the neural state of the subject based on determined anomalies in the recorded compound action potential.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of Australian Provisional PatentApplication No. 2014901110 filed 28 Mar. 2014, which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to assessing a neural state from neuralpotentials, and in particular relates to obtaining a recording of aneural potential arising on neural tissue, and monitoring for ananomalous profile of the recording, in order to assess the existence,state or progress of a neural disease.

BACKGROUND OF THE INVENTION

Neuropathic pain arises from damage or disease affecting thesomatosensory system, and may result from disorders of the peripheralnervous system or the central nervous system. For example, complexregional pain syndrome (CRPS) is a severe type of pain disorder.

There is no known single pathognomonic symptom or sign of neuropathicdisease. Consequently, it is difficult to diagnose neuropathic diseaseand to monitor the progress of neuropathic disease. No conclusiveobjective diagnostic exists for neuropathic pain, and clinicians mustrely largely on a subjective clinical observation of the patient'sresponses. Neuropathic pain is also difficult to treat and oftenresponds poorly to standard pain treatments.

A range of medications for treating neuropathic pain exist, includinggabapentin for example. Careful documentation and appropriate monitoringof treatment are important for the safe and effective use of suchmedications, however this is difficult to achieve due to the difficultyof determining the disease state or monitoring the progress of thedisease or symptoms. Advanced therapies for treating neuropathic paininclude spinal cord stimulation.

Any discussion of documents, acts, materials, devices, articles or thelike which has been included in the present specification is solely forthe purpose of providing a context for the present invention. It is notto be taken as an admission that any or all of these matters form partof the prior art base or were common general knowledge in the fieldrelevant to the present invention as it existed before the priority dateof each claim of this application.

Throughout this specification the word “comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusionof a stated element, integer or step, or group of elements, integers orsteps, but not the exclusion of any other element, integer or step, orgroup of elements, integers or steps.

In this specification, a statement that an element may be “at least oneof” a list of options is to be understood that the element may be anyone of the listed options, or may be any combination of two or more ofthe listed options.

SUMMARY OF THE INVENTION

According to a first aspect the present invention provides a method ofassessing a neural state of a subject, the method comprising:

obtaining a recording of a compound action potential arising in neuraltissue of the subject;

processing the recording to determine whether a profile of the recordedcompound action potential is anomalous; and

outputting an indication regarding the neural state of the subject basedon determined anomalies in the recorded compound action potential.

A method for determining whether a human patient has neuropathicdisease, comprising:

obtaining a recording of a compound action potential arising in neuraltissue of the patient; and

diagnosing the patient as having neuropathic disease if a profile of therecorded compound action potential is anomalous.

A non-transitory computer readable medium for assessing a neural stateof a subject, comprising instructions which, when executed by one ormore processors, causes performance of the following:

obtaining a recording of a compound action potential arising in neuraltissue of the subject;

processing the recording to determine whether a profile of the recordedcompound action potential is anomalous; and

outputting an indication regarding the neural state of the subject basedon determined anomalies in the recorded compound action potential.

The detection of irregularities or anomalies in the recorded responsemay comprise any one or more of:

determining whether more than three peaks exist in the recorded compoundaction potential;

determining whether a peak in the recorded compound action potential isunexpectedly broad;

determining whether a peak in the recorded compound action potential hasan atypically swift rate of rise;

determining whether anomalous frequency components exist in the recordedcompound action potential when assessed in the frequency domain;

determining a degree of deviation of the recorded compound actionpotential from a predefined expected response profile and, if the degreeof deviation exceeds a predetermined threshold, indicating that therecorded response is anomalous.

Some embodiments may determine whether more than three peaks exist inthe recorded compound action potential by measuring an amplitude orpower of the recorded compound action potential in a time windowpositioned after cessation of a normal response. The amplitude or powerof the recorded compound action potential in such a time window can beused to assess the presence or absence of an abnormal response arisinglater than a normal P2 peak. Additionally or alternatively, a matchedfilter or other signal processing means may be used to detect thepresence of an extra lobe in the recorded compound action potential.

Some embodiments of the present invention thus recognise that whenconsidering a recorded compound action potential (CAP) obtained from aperson suffering from an altered neural state such as CRPS, rather thanthe CAP taking a typical three lobed profile, lobe deformation oradditional lobes referred to herein as doublets can be observed to arisein the ECAP. Moreover, the degree of lobe deformation and/or therelative size of the additional lobes appearing in the response can bemeasured, in order to give not only a binary diagnosis but also aquantitative measure of the severity of the disease suffered by theperson. Absence of such response profile anomalies may be used toeliminate some diseases from a diagnosis for the person. Repeatedassessment of the recorded response profile from time to time, forexample throughout administration of a therapy, may be used to assessdisease state, disease progress, and therapy efficacy, and may be usedto guide therapy modifications and optimisation over time. Therapymodifications may include modifications of dosage of a medicament and/ormodification of a stimulus regime applied by a spinal column stimulator.

Accordingly, the present invention recognises that monitoring for theoccurrence and severity of anomalies such as doublets in the recordedresponse profile gives a diagnostic for neuropathic pain or neuraldamage or in general any neural disease which gives rise to atypicalneural response profiles.

Notably, some embodiments of the present invention further recognisethat when application of a stimulus to a first neural site gives rise toanomalies in a recorded neural response profile, application of the samestimulus to an alternative neural site might give rise to a recordedneural response without abnormalities. Such embodiments may thus providefor identifying a locus of neuropathic pain.

The method of the present invention may in some embodiments be performedintra-operatively for example to effect electrode array implantationsite optimisation. The method of the present invention may additionallyor alternatively be performed during an implant programming stage inorder to optimise electrode selection to a site at which a locus ofneuropathic pain is identified.

The invention may further provide for intra-operative monitoring of theresponse profile during a sympathectomy procedure, in order to providean intra-operative progressive indication of efficacy of thesympathectomy.

According to a further aspect the present invention provides a method oftreating a neural disease, the method comprising:

ordering or requesting the result of the method of the first aspect; and

administering or modifying a therapy in a manner responsive to theordered result.

The compound action potential may arise from deliberate stimulation,whether peripheral stimulation or direct spinal column stimulation, forexample.

BRIEF DESCRIPTION OF THE DRAWINGS

An example of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1a schematically illustrates an implanted spinal cord stimulatorsuitable for implementing the present invention;

FIG. 1b is a block diagram of the implanted neurostimulator;

FIG. 1c is a schematic illustrating interaction of the implantedstimulator with a nerve;

FIG. 2a illustrates the typical form of an electrically evoked compoundaction potential of a healthy subject, and FIGS. 2b and 2c illustratehow the CAP manifests in the recording when using a differentialrecording arrangement with an epidural ground;

FIG. 3 illustrates an actual ECAP recording obtained from a subjecthaving a normal neural state;

FIG. 4 illustrates anomalous ECAP recordings obtained from a subjectsuffering a neural disease;

FIG. 5 illustrates anomalous ECAP recordings obtained from anothersubject suffering a neural disease;

FIG. 6 is a plot of the differences between the N1, N2 peaks measureddoublets;

FIG. 7 shows the normalised antidromic responses from three patientsplotted together;

FIG. 8 shows an example of a large doublet response in the antidromicresponse of one patient;

FIG. 9 is a plot of the normalized masker probe results for therefractory period of three patients;

FIGS. 10-12 illustrate the relative severity of doublet formation forthree respective patients; and

FIG. 13 illustrates a control system by which a therapy may be modifiedin accordance with one embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates an implanted spinal cord stimulator 100suitable for implementing the present invention. Stimulator 100comprises an electronics module 110 implanted at a suitable location inthe patient's lower abdominal area or posterior superior gluteal region,and an electrode assembly 150 implanted within the epidural space andconnected to the module 110 by a suitable lead. Numerous aspects ofoperation of implanted neural device 100 are reconfigurable by anexternal control device 192. Moreover, implanted neural device 100serves a data gathering role, with gathered data being communicated toexternal device 192.

FIG. 1b is a block diagram of the implanted neurostimulator 100. Module110 contains a battery 112 and a telemetry module 114. In embodiments ofthe present invention, any suitable type of transcutaneous communication190, such as infrared (IR), electromagnetic, capacitive and inductivetransfer, may be used by telemetry module 114 to transfer power and/ordata between an external device 192 and the electronics module 110.

Module controller 116 has an associated memory 118 storing patientsettings 120, control programs 122 and the like. Controller 116 controlsa pulse generator 124 to generate stimuli in the form of current pulsesin accordance with the patient settings 120 and control programs 122.Electrode selection module 126 switches the generated pulses to theappropriate electrode(s) of electrode array 150, for delivery of thecurrent pulse to the tissue surrounding the selected electrode(s).Measurement circuitry 128 is configured to capture measurements ofneural responses sensed at sense electrode(s) of the electrode array asselected by electrode selection module 126.

FIG. 1c is a schematic illustrating interaction of the implantedstimulator 100 with a nerve 180, in this case the spinal cord howeveralternative embodiments may be positioned adjacent any desired neuraltissue including a peripheral nerve, visceral nerve, parasympatheticnerve or a brain structure. Electrode selection module 126 selects astimulation electrode 2 of electrode array 150 to deliver an electricalcurrent pulse to surrounding tissue including nerve 180, and alsoselects a return electrode 4 of the array 150 for stimulus currentrecovery to maintain a zero net charge transfer.

Delivery of an appropriate stimulus to the nerve 180 evokes a neuralresponse comprising a compound action potential which will propagatealong the nerve 180 as illustrated, for therapeutic purposes which inthe case of a spinal cord stimulator for chronic pain might be to createparaesthesia at a desired location. To this end the stimulus electrodesare used to deliver stimuli at 30 Hz. To fit the device, a clinicianapplies stimuli which produce a sensation that is experienced by theuser as a paraesthesia. When the paraesthesia is in a location and of asize which is congruent with the area of the user's body affected bypain, the clinician nominates that configuration for ongoing use.

The device 100 is further configured to sense the existence andintensity of compound action potentials (CAPs) propagating along nerve180, whether such CAPs are evoked by the stimulus from electrodes 2 and4, or otherwise evoked. To this end, any electrodes of the array 150 maybe selected by the electrode selection module 126 to serve asmeasurement electrode 6 and measurement reference electrode 8. Signalssensed by the measurement electrodes 6 and 8 are passed to measurementcircuitry 128, which for example may operate in accordance with theteachings of International Patent Application Publication No.WO2012155183 by the present applicant, the content of which isincorporated herein by reference.

FIG. 2a illustrates the typical form of an electrically evoked compoundaction potential of a healthy subject. The shape of the compound actionpotential shown in FIG. 2a is predictable because it is a result of theion currents produced by the ensemble of axons generating actionpotentials in response to stimulation. The action potentials generatedamong a large number of fibres sum to form a compound action potential(CAP). The CAP is the sum of responses from a large number of singlefibre action potentials. The CAP recorded is the result of a largenumber of different fibres depolarising. The propagation velocity isdetermined largely by the fibre diameter. The CAP generated from thefiring of a group of similar fibres is measured as a positive peakpotential P1, then a negative peak N1, followed by a second positivepeak P2. This is caused by the region of activation passing therecording electrode as the action potentials propagate along theindividual fibres. An observed CAP signal will typically have a maximumamplitude in the range of microvolts.

The CAP profile takes a typical form and can be characterised by anysuitable parameter(s) of which some are indicated in FIG. 2a . Thepositions and amplitudes of the peaks can for example be used alone orin combination to generate a correlation between them and the state andseverity of a central nervous system (CNS) disorder. Depending on thepolarity of recording, a normal recorded profile may take an inverseform to that shown in FIG. 2a , i.e. having two negative peaks N1 andN2, and one positive peak P1.

FIG. 2b illustrates how the CAP manifests in the recording, when using adifferential recording arrangement with an epidural ground. In FIG. 2b anormal ECAP shape (A) is inverted and delayed by the propagationdistance to the epidural ground electrode (B), and so the differentialmeasure will look like the envelope of C. FIG. 2c shows thecorresponding manifestation in relation to an anomalous CAP (D). Theanomalous CAP has a strong doublet, which is inverted and delayed by thepropagation distance to the epidural ground electrode (E), and so thedifferential measure will look like the envelope of F. As shown in FIG.2c , and also being the case for FIG. 2b , the actual recording obtainedtypically does not include the first positive peak as it is obscured bythe stimulus.

The present invention thus recognises that the shape or profile of thecompound action potential reflects changes in the ion channelcharacteristics as a result of pathological or natural change.

EXAMPLES

Comparison of ECAP measurements from the dorsal column of a number ofdifferent human subjects was undertaken in order to identify systematicdifferences which relate to either genetic or pathological differencesbetween subjects. Measurements of dorsal column evoked compound actionpotentials show distinct differences between the ECAP shapes measured atdifferent electrodes along the array.

FIG. 3 shows a “normal” ECAP, being a triphasic P1, N1, P2 response, asobtained from “patient 25”. The use of epidural ground inverts the N1 ata time when the response passes the ground electrode. As the recordedresponse of FIG. 3 exhibits no significant abnormalities as compared tothe predicted response of FIG. 2, Patient 25 can be diagnosed as havingno measurable neuropathic disease.

In contrast, FIG. 4 shows data from patient 34, measured in both theorthodromic and antidromic directions at respective electrodes eitherside of the stimulus electrode, each spaced apart from the stimuluselectrode by three electrodes. The N1 peak 402 is broader in theorthodromic direction, displays a faster rise time and is larger inamplitude. Moreover, an additional lobe 404 has emerged in theorthodromic response, in deviation from the expected response of FIG. 3.Any or all of these abnormalities may be detected and/or quantified inorder to produce an automated diagnosis of the existence or severity ofneural disease in patient 34. For example in some embodiments ameasurement may be taken of the signal amplitude or power occurringwithin a time window covering the anomalous peak 404. When the amplitudeor power in such a time window exceeds a threshold the response may beflagged as being anomalous.

FIG. 5 illustrates the recordings of the corresponding orthodromic andantidromic responses arising from patient 22. As seen at 502 in the N1peak of the orthodromic response, the N1 peak 502 is broader in theorthodromic direction, displays a faster rise time and is larger inamplitude. An additional lobe 504 has emerged in the orthodromicresponse, in deviation from the expected response of FIG. 3. Thuspatient 22 exhibits doublets which may be detected and/or quantified inorder to produce an automated diagnosis of the existence or severity ofneural disease in patient 22.

FIG. 6 is a histogram of N1 peak latencies in ms, measured at the samestimulus electrode to recording electrode separation, for a large numberof patients. This illustrates that N1 peak latency is predictable withinquite a narrow time range as the peaks have quite a narrow spread over alarge number of patients.

FIG. 7 shows the normalised antidromic responses from three patientsplotted together. The N1 peaks have very similar latencies. The peakshapes 702 and 704 are normal, noting the effects described in relationto FIGS. 2b and 2 c.

FIG. 8 shows an example of a large doublet response in the antidromicresponse of one patient, illustrating that severity of the neural statecan be distinguished, for example by comparing the normalised height oflobe 804 to say lobe 404 or 504.

To explore the question of ectopic discharge, the refractory period wasinvestigated using the “masker probe” techniques set forth inInternational Patent Application Publication No. WO2012/155189, thecontents of which are incorporated herein by reference. FIG. 9 is a plotof the normalized masker probe results for 3 patients, denoted patientnos 16, 19 and 35 respectively. For patient 35 the masked amplitude wasdivided by the unmasked amplitude. To allow for differences in themeasurement mode for patients 16 and 19, the results were normalizedagainst the responses at ˜5000 micro seconds inter-stimulus interval(ISI). In general the results are consistent between patients. As shownin FIGS. 10-12, the CAP profile of patient 35 had the largest doublepeaks or doublets of the three patients, and also at short ISI's of theorder of 100-200 us patient 35 had the largest additional recruitment asindicated at 902. The data for patient 16 was collected with an 80 uspulse width, and so this will affect the additional recruitment at theshort ISI's.

FIG. 10 illustrates the progression of CAP profile as the CAP travelsaway from the stimulus site, for patient 35. This indicates that theexistence of an atypical CAP profile may best be detected by makingrecordings very close to the stimulus site. It is noted that theanomalous peaks propagate with distance, which indicates that they areneural responses from the same group or class of fibres. FIG. 11 shows aresponse obtained from patient 16, and FIG. 12 shows a response obtainedfrom patient 19, revealing that of these three patients Patient 35 hasthe most severe doublet formation in their neural response.

There appears to be little consistency between the N1 latency and theappearance of the double response so N1 latency may not be a suitableparameter for diagnosing neural state.

Some embodiments may provide for repeated assessment of the recordedresponse profile from time to time, for example throughoutadministration of a therapy, in order to assess disease state, diseaseprogress, and therapy efficacy, and may be used to guide therapymodifications and optimisation over time. Therapy modifications mayinclude modifications of dosage of a medicament and/or modification of astimulus regime applied by a spinal column stimulator. FIG. 13illustrates a control loop by which drug dosage or electrical stimulidosage is adjusted in a dynamic manner, with the magnitude of thedoublet (404, 504) being used as a control variable for a feedback loop.

It will be appreciated by persons skilled in the art that numerousvariations and/or modifications may be made to the invention as shown inthe specific embodiments without departing from the spirit or scope ofthe invention as broadly described. The present embodiments are,therefore, to be considered in all respects as illustrative and notrestrictive.

1. A method of assessing a neural state of a subject, the method comprising: obtaining a recording of a compound action potential arising in neural tissue of the subject; processing the recording to determine whether a profile of the recorded compound action potential is anomalous; and outputting an indication regarding the neural state of the subject based on determined anomalies in the recorded compound action potential.
 2. The method of claim 1 wherein the detection of anomalies in the recorded response comprises determining whether more than three peaks exist in the recorded compound action potential.
 3. The method of claim 1 wherein the detection of anomalies in the recorded response comprises determining whether a peak in the recorded compound action potential is unexpectedly broad.
 4. The method of claim 1 wherein the detection of anomalies in the recorded response comprises determining whether a peak in the recorded compound action potential has an atypically swift rate of rise.
 5. The method of claim 1 wherein the detection of anomalies in the recorded response comprises determining whether anomalous frequency components exist in the recorded compound action potential when assessed in the frequency domain.
 6. The method of claim 1 wherein the detection of anomalies in the recorded response comprises determining a degree of deviation of the recorded compound action potential from a predefined expected response profile and, if the degree of deviation exceeds a predetermined threshold, indicating that the recorded response is anomalous.
 7. The method of claim 1 wherein the detection of anomalies in the recorded response comprises identifying a locus of neuropathic pain by applying stimuli to first and second neural sites and determining which stimulus gives rise to greatest anomalies in a recorded neural response profile.
 8. The method of claim 1 when performed intra-operatively to affect effect electrode array implantation site optimisation.
 9. The method of claim 1 when performed during an implant programming stage in order to optimise electrode selection.
 10. The method of claim 1 when performed intra-operatively during a sympathectomy procedure, in order to provide an intra-operative progressive indication of efficacy of the sympathectomy.
 11. A method of treating a neural disease, the method comprising: ordering or requesting the result of the method of claim 1; and administering or modifying a therapy in a manner responsive to the ordered result.
 12. A method for determining whether a human patient has neuropathic disease, comprising: obtaining a recording of a compound action potential arising in neural tissue of the patient; and diagnosing the patient as having neuropathic disease if a profile of the recorded compound action potential is anomalous.
 13. A non-transitory computer readable medium for assessing a neural state of a subject, comprising instructions which, when executed by one or more processors, causes performance of the following: obtaining a recording of a compound action potential arising in neural tissue of the subject; processing the recording to determine whether a profile of the recorded compound action potential is anomalous; and outputting an indication regarding the neural state of the subject based on determined anomalies in the recorded compound action potential. 